1. Field of the Invention
The present invention concerns machines for the implantation of cathodes in cathode-ray tube guns, and more precisely to an improvement for a machine described in the copending application having Ser. No. 07/169,952 filed Mar. 18, 1988 on behalf of the applicant, now U.S. Pat. No. 4,850,920.
2. Description of the Prior Art
Machines currently used to insert cathodes in the electron guns of cathode-ray tubes include, for the purpose of measuring the distance between the active face of the cathode and the gird G1 or, as the case may be, the grid G2, complicated instruments which are slow and hardly maintain the requisite precision.
For, the machines position the cathode with respect to the first grid, or grid G1, by first interposing a sensor with a fixed length between the cathode and the grid G1, said cathode being mounted on a fixed support and the gun being mounted on a moving support driven by a wheel that works in cooperation with the cam-shaped groove of a leading screw which is itself driven by a stepping motor.
A first level or shoulder of this cam-shaped groove defines the measurement position and the positional setting of the cathode. Then the gun is withdrawn, the sensor is released, and the gun goes forward by a distance equal to the distance by which it has been withdrawn, plus the length of the sensor. Thus we come to a position in which the cathode is soldered into the eyelet of the gun, this position being determined also by a shoulder of the groove. Thus, any modification in the setting of the machine means that the sensor has to be dismantled and re-machined to new dimensions. Furthermore, the feed device formed by a leading screw working together with a wheel, has little precision: the groove of the leading screw has to be free of all dust or debris, and this is difficult to obtain in an industrial environment. Moreover, even if the step motor stops precisely in the desired angular position, the coupling device with the leading screw that drives it has backlashes which are difficult to take into account. The stresses exerted on the wheel, as much by the gun carriage as by the leading screw, mean that the backlashes in this transmission of motion are reflected at the carriage. Furthermore, if the cathode eyelet is out of ine, there is a risk that the sensor will deform the eyelet, or else the sensor will have to be made of a deformable material.
To overcome these drawbacks, the French patent No. 87 03922 has proposed a machine for implanting a cathode in an eyelet of an electron gun, comprising a movable gun supporting device that moves in the direction of its axis, said device being movable between two fixed positions, and a sensor with a rectractile rod which can be moved perpendicularly to the axis of the gun.
In the case of the above-described implanting machine, when the gun-supporting device is placed in the position where it is at the furthest distance from the cathode, the sensor with a rectractile rod is inserted between the gun and the cathode. The rod of the sensor is then brought to a stop against the grid G1, and it is thus possible to define the distance between the sensor and the active face of the cathode. Thus, the active face of the cathode is positioned in such a way that when the sensor is removed and the gun-supporting device is brought to its second fixed position, the desired distance is obtained between the active face of the cathode and the grid G1. This machine for implanting a cathode in the eyelet of an electron gun thus enables precise positioning of the cathode with respect to the grid G1. However, it takes into account neither the position of the grid G2 nor the spacing between the grid G1 and the grid G2. Now, to obtain accurate functioning of a cathode-ray tube, the distance between the grids G1 and G2 should range between specific values so as to give a precise cut-off voltage.
It is an object of the invention is to overcome these drawbacks in proposing a cathode implanting machine for implanting a cathode in a cathode-ray tube gun eyelet making it possible to achieve, in one operation, the measurement of position of the grid G1, the grid G2 and the cathode.